Alibaba Group Holding Ltd. unveiled plans for an initial public offering that values the company at $155 billion, which would instantly make it one of the largest listed in the U.S. and mark one of the biggest stock-market debuts ever.

The Chinese e-commerce company, in papers filed Friday, valued itself just below rival Amazon.com Inc.'s $160 billion market capitalization. However, Alibaba's worth could rise in the days ahead as the company officially introduces itself to investors in a process known as a roadshow, set to...